The present invention relates to a semiconductor device and a method of manufacturing thereof.
A ferroelectric random access memory (FeRAM) is a kind of nonvolatile memory which uses in its capacitor part a ferroelectric film such as PZT (Pb(ZrxTi1−x)O3), BIT (Bi4Ti3O12), SBT (SrBi2Ta2O9), or the like, of which residual polarization is used in holding data.
As a method of forming a PZT film to serve as a ferroelectric film on a lower electrode, there is a sputtering method. In this method, an amorphous PZT film is formed on the lower electrode by sputtering, after which a thermal process using RTA (rapid thermal annealing) is carried out in an oxygen stream environment at a temperature of 650° C. to crystallize the amorphous PZT film.
With respect to such PZT film, however, defects such as oxygen vacancies can be generated easily at the interface portion with the lower electrode. Such defects will generate space charge which could deteriorate the fatigue characteristic (i.e. a deterioration behavior in the amount of polarization due to polarization inversion), the imprint characteristic (i.e. a phenomenon in which, when polarization is turned to one direction and retained, the polarization becomes more tended to turn in that direction), and so forth.
As a method to cope with such characteristic deterioration, a ferroelectric capacitor having a ferroelectric film (PZT film) being formed by a MOCVD (Metal Organic Chemical Vapor Deposition) method on a lower electrode that has a laminated structure composed of Pt and a conductive oxide film such as SRO (SrRuO3) has been proposed. By using such conductive oxide film, switching voltage can be reduced, enabling to bring out better switching characteristic, and in addition to that, the fatigue characteristic can be improved.
However, the PZT film formed on the conductive oxide film using the MOCVD method will have lower crystallizing ability, which allows defects such as vacancies to be formed inside the film more easily. This leads to a problem of decreasing reliability of a semiconductor device.
Meanwhile, along with the FeRAM becoming more highly integrated, there is a need for miniaturization of capacitor size. As one method of miniaturizing the capacitor size, thinning of a capacitor film, especially of a lower electrode film, can be considered.
The lower electrode film is usually formed by a noble metal film such as Pt, Ir, or the like, for instance, and it has influence on the structural characteristic and the electrical characteristic of the ferroelectric film formed thereon. By reducing the film thickness of the lower electrode film, it is possible to prevent the capacitor size from expanding in the horizontal direction at the time of processing the capacitor using RIE (reactive ion etching) with a taper angle.
However, thinning the lower electrode film (noble metal film) will result in deterioration in the crystallizing ability, orientation intensity, etc. of the lower electrode film. Therefore, with this method, the crystallizing ability, the orientation and the electrical characteristic of the ferroelectric film formed on the lower electrode film will deteriorate, which leads to a problem of decreasing reliability of a semiconductor device.
As another method of miniaturizing the capacitor size, a three-dimensional structured capacitor, which is a capacitor being three-dimensionalized, can be considered. As to the types of the three-dimensional structured capacitors, there are a concave type, a convex type, and so forth. In the concave type capacitor, a capacitor area is dug in a shape of a trench inside which a ferroelectric film is formed. In the convex type capacitor, a ferroelectric film is formed on a lower electrode having a convex shape.
In a case when a method of producing the lower electrode with a laminated structure of a noble metal film such as Pt and a conductive oxide film such as SRO, as the one described above, is applied to the three-dimensional structured capacitor (especially the concave type), it should be difficult to form the conductive oxide film with uniform composition and even thickness on the noble metal film. Such possible changes in composition and film thickness lead to a problem of deteriorating the capacitor characteristic.
In this way, in the case of thinning the lower electrode film or three-dimensionalizing the capacitor for the purpose of miniaturizing the capacitor size, there is a problem that reliability of a semiconductor device will be deteriorated.